A transport device (or paper feed device) of a printer transfers a transporting force to a recording medium such as paper, etc., by transmitting the driving force generated by a drive source such as a motor, etc., via multiple gears to transport rollers and rotating the transport rollers.
If for any reason the recording medium should become jammed while being transported, it is necessary to pause the transport rollers (the transport device) and remove the recording medium that has become jammed. In these situations, the recording medium is ordinarily pressed against the transport rollers. So, when the recording medium is pulled to remove the recording medium, the recording medium engages with the transport rollers and forces them to rotate as the recording medium is forcibly removed.
At this time, when the rotation of the transport rollers is transmitted via the gears to the motor (that is the drive source), the force required to rotate the transport rollers becomes extremely large, and a large force becomes necessary in order to remove the recording medium. So the workability of the operation to remove the recording medium that has become jammed deteriorates remarkably.
With this difficulty of conventional systems, an intermediate gear (a planetary gear) that engages with a driving gear (a sun gear) has been used so that it can rotate (and/or oscillate) in relation to the center of rotation of the driving gear. The driving force of the driving gear is transmitted via this intermediate gear to the driven gear in order to transport the recording medium. If for any reason the paper should become jammed, then the driving gear is rotated in reverse. The intermediate gear uses the reaction force received from the driving gear to rotate (and/or oscillate) the intermediate gear in relation to the center of rotation of the driving gear, such that the engaged state of the driven gear and the intermediate gear is released, preventing the rotation from being transmitted from the driven gear (driven by the force associated with the removal the recording medium) to the driving gear.
Incidentally, the intermediate gear receives a reaction force from the driven gear as well, and the direction of this reaction force—in an involute gear, for example—becomes the direction of the angle formed by a tangent line that passes through the pitch point where the pitch circle and the involute tooth intersect and a perpendicular line erected at the involute tooth that passes through this pitch point, or in other words, the direction of the pressure angle at the pitch point.
Also, the reaction force F from the driven gear 103 is ultimately received at the rotation axis 102c of the intermediate gear 102 as shown in FIG. 12. When the pressure angle is made to be 20°, a first center line L1 is made to connect the center of rotation O2 of the intermediate gear 102 and the center of rotation O1 of the driving gear 101, and a second center line L is made to connect the center of rotation O2 of the intermediate gear 102 and the center of rotation O3 of the driven gear 103. When the angle formed by the first center line L1 and the second center line L2 (hereinafter this angle is called the arrangement angle θ) is less than 110°, the force in the direction of releasing the engaged state of the driven gear 103 and the intermediate gear 102 acts on the rotation axis 102c (intermediate gear 102) due to the Fr component of reaction force F.
For this reason, when the arrangement angle θ is less than 110°, when the driving gear is rotated normally and the driving force is transmitted to the driven gear, the engagement of the intermediate gear and the driven gear is released, so it becomes impossible to transmit the driving force to the driven gear.
Correspondingly, it is acceptable to make the arrangement angle θ greater than or equal to 110°, but when the arrangement angle θ is increased, it is highly likely that a new problem will arise, namely that the driven gear or the driving gear will interfere with the other items.
Incidentally, if the arrangement angle θ is made to be 110° then the Fr component is no longer produced. Also, if the arrangement angle θ is made to be greater than or equal to 110°, then the direction of the Fr component becomes the opposite of that in FIG. 12, so the force in the direction of releasing the engaged state of the driven gear 103 and the intermediate gear 102 no longer acts on the rotation axis 102c (intermediate gear 102).